The action of many DNA cleaving drugs is based on the formation of a reactive polyatomic mono- or biradical that then abstracts a hydrogen atom from a sugar moiety in DNA, inducing a strand cleavage. Understanding the factors that control the efficiency of his hydrogen atom abstraction reaction is crucial for strategic antitumor and antiviral drug design. However, little is currently known about these factors or about the properties of the reactive radical intermediates formed from drugs. This study addresses these issues by using mass spectrometry experiments to examine the intrinsic reactivity of differently substituted phenyl radicals and biradicals pertinent to the action of antitumor and antiviral (AIDS) drugs. The structural features are explored that determine a radicals ability to abstract a hydrogen atom from deoxyribose, its selectivity toward different hydrogen atoms, and its reactivity toward different substances. Solvent effects are addressed by examining reactions as a function of stepwise solvation. The experimental data is complemented by thermochemical information obtained from molecular orbital calculations. In addition to examination of the factors that control the reactivity of known types of nonhydrolytic DNA cleavers, various novel mono-, bi- and triradicals are screened for their ability to abstract a hydrogen atom from DNA. The goal is to discover efficient DNA cleavers that may provide the basis for the development of new types of pharmaceuticals. Further, the use of photolysis to generate DNA-cleaving radicals is explored as an effort to advance the design of artificial DNA photo cleaving agents for site-specific medical treatments. Finally, different methods to trap radicals are evaluated for possible application in prevention of free radical damage caused by such conditions as Alzheimer s disease and strokes.